Adjustable thermostat

ABSTRACT

An adjustable snap disc thermostat is disclosed which utilizes a bimetallic snap disc both for thermal response and snap action. The disc operates a single pole double throw switch only while the disc is in full snap movement. Therefore, full contact pressure is maintained until the instant the switch opens. Temperature adjustment is provided by a leaf spring pivotally supported at one end on the body and having an operating portion at the other end which urges the disc toward one position of stability. An adjusting screw engages the leaf spring at a location relatively close to the pivot end thereof so that excessive adjustment movement is not required for substantial force adjustment. Also, such spring provides a relatively low spring rate through the range of snap travel.

United States Patent [72] Inventor Donald E. Place Mansfield, Ohio [211 Appl. No. 41,910 [22] Filed June 1, 1970 [45] Patented Aug. 31, 1971 [73] Assignee Tberm-O-Disc lnc.

Mansfield, Ohio 54 ADJUSTABLE THERMOSTAT 18 Claims, 5 Drawing Figs.

[52] US. 337/349, 337/91, 337/354, 337/367 [51] Int. Cl H0111 37/52 [50] FieldofSeareh 337/91, 57, 93, 94, 347, 349, 357, 360, 361, 368, 374, 375, 367, 354

[56] References Cited UNITED STATES PATENTS 2,312,948 3/1943 Winborne et a1. 337/348 2,585,340 2/1952 Miller 337/347 X 3,164,701 1/1965 Kirchhubel. 337/347 X 3,500,278 3/1970 Them 337/367 X Primary Examiner-Bernard A. Gilheany Assistant Examiner-Dewitt M. Morgan AttameyMcNenny, Farrington, Peame & Gordon ABSTRACT: An adjustable snap disc thermostat is disclosed which utilizes a bimetallic snap disc both for thermal response and snap action. The disc operates a single pole double throw switch only while the disc is in full snap movement. Therefore, full contact pressure is maintained until the instant the switch opens. Temperature adjustment is provided by a leaf spring pivotally supported at one end on. the body and having an operating portion at the other end which urges the disc toward one position of stability. An adjusting screw engages the leaf spring at a location relatively close to the pivot end thereof so that excessive adjustment movement is not required for substantial force adjustment. Also, such spring provides a relatively low spring rate through the range of snap travel.

ADJUSTABLE rmmmosrA'r BACKGROUND or THE INVENTION This invention relates generally to thermostats and more particularly to a novel and improved adjustable thermostat which is operable by a bimetallic snap disc.

The pending application of Donald -J. Schmitt, Ser. No. 779,680, filediNov. 29,-1968, assignedto theassignee of the present invention, disclosesembodiments of.an adjustable thermostat incorporating a bimetallic snap disc to provide both the" snap action and the thermal response. Reference to thatapplication should be ,made for a discussion of the mode of operation of such disc and the manner in which the operating temperatures of such disc may be modified or adjusted by applying biasing forces to the disc.

v I PRIOR ART In the past is has: been customary, in most instances, to utilizetbimetallic blades-which operate a snap switch when a bimetallic operated thermostat requires adjustment of operating,temperature. Examples of such adjustablethermostats are disclosed in the U.S. Pat. Nos. 2,645,692; Re 26,554; and 3,462,772.: Although some early patents disclose bimetallic snap disc thermostats which provide temperature adjustment (for example, U.S.- ,Pat. Nos. 1,958,594; 1,972,832; and 2,153,297) in recent years the vast majority of bimetallic snap disc thermostats havenot been adjustable, butrather have operated at predetermined temperatures that are determined by themanufacture of the disc itself. Examples of such nonadjustable thermostats are disclosed in the U.S. Pat. Nos. 2,717,936; 3,081,388; and 3,470,517.

1 SUMMARY OF THE INVENTION The present invention:providesasimple and reliable adjustablejthermostatincorporating a bimetallic snap disc. The illustrated embodiment is particularly suited for automatically controllingthe operation. of electric hot water heaters or the like. Such embodiment is suitable for repeatedly cycling the electric heaters in such hot water heatingtanks and provides temperature accuracy and operating reliability for extended periods even when the thermostat is used to control high capacity heaters whichrequire the initiation and interruption of large currents. Iri the illustrated embodiment the bimetallic sriapdisc. is at, all times seated around its entire. periphery against a metallic cover' element so direct conductive type heat exchange is provided between the cover and the disc. The

metal cover is preferably positioned against the metal of the tank to insure rapid and accurate response.

' A leaf spring is supported at one end by the thermostat body by a knife-edge-type-pivot connection and is provided at its opposite end with a 1 free end portion which engages the bumper so that the spring force is applied substantially along the;center-line of the disc. The force of the leaf spring on the discjsadjusted by anadjusting screw. The various elements areaproportioned. to minimize any friction inthe spring system and thereby.,minimi zeany operating temperature inaccuracy which would result if significant friction were present.

The location of the adjustment screw is substantially adjacent to the-pivotal mounting of the spring and a substantial distance from the free end of the spring. This provides subs'tantial adjustment of the force of the spring on the disc with a relatively small movementof the adjusting screw. With this structure the leaf spring-has arelatively low rate with respect to movement of the free end thereof. 1 v

* The disc is freeof positive restraint which would limit the disc travel. In theillustrated embodiment, the switch is a double throw switch and the fixed contact remote from the disc is supported byaspring arm so that it doesnot limit the disc travel. An adjustablestop is provided to limitthe movement of this, contact'toward the disc to insure that the contacts open properly and to provide adjustment of the spacing between the two fixed contacts.

These and 'other features of this invention, which are discussed in considerable detail below, result in a novel and improved structurally simple, accurate, and reliable thermostat.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view of the front face of a thermostat incorporating this invention;

FIG. 2 is a side view;

FIG. 3 is a back view;

FIG. 4 is a longitudinal section taken generally along 4-4 of FIG. 1; and

FIG. 5 is a fragmentary section taken generally along 5-5 of FIG. 4 with the cover removed.

DETAILED DESCRIPTION OF THE DRAWINGS The illustrated embodiment of this invention is particularly suited for use in controlling the operation of an electric hot water heater or the like. In such systems it is customary to provide an upper heater located near the top of the tank and a separate or lower heater located adjacent to the bottom of the tank. In such systems an upper:thermo stat having a double throw switch is usually positioned adjacent to the upper heater and is connected to control the operation thereof. A similar, but usually single pole, single throw thermostat, is also normally mounted adjacent to the lower heater for control of the lower heater. It should be understood, however, that although the illustrated embodiment of this invention is particularly suited for use in the controlling of electric hot water heaters, such thermostats and other embodiments of thermostats incorporating this invention can be used for many other purposes with equal facility. 1

Referring to FIGS. 1 through 5, the thermostat includes a body assembly 10 consisting of three basic parts: a main body member 11, a cover member 12, both of which are preferably formed of a molded plastic material such as a phenolic resin, and a metallic cover and mounting plate 13 which is preferably a stamping. The main body member 11 and cover member 12 cooperate to define a switch cavity 14 which encloses the thermostat switch. A disc chamber 16 separated from the switch cavity 14 -by a wall 17 in the main body member 11 is enclosed by the main body member 11 in cooperation with the cover and mounting plate 13.

The thermostat switch mechanism includes a first fixed contact 18 secured to a fixed contact support plate 19 which is, in turn, secured against one face 21 of the'wall 17 by a rivet type fastener 22. The support plate 19 is provided with a lateral extension which projects out of the switch chamber 14 and functions as a terminal 23 which is threaded to receive a terminal screw 24.

A second fixed contact 26 is mounted on a spring arm 27 in a position overlying the first contact 18. The opposite end of the spring arm 27 is secured to themain body member 10 by a rivet 28. Extending along the upper side of the spring arm 27 is a plate 29 which is preferably permanently connected to the spring arm by spot welds at 31. The plate 29 is provided with a lateral projection which extends out of the switch chamber 14 and provides a terminal 32 which is threaded to receive a terminal screw 33. Positioned'adjacent to the side of the spring arm 27 opposite the plate 29 is a stop strap 34 which is also secured to the spring arm by the spot welds at 31. The stop strap 34 extends along the spring arm 27 in spaced relationship with respect thereto to an end 36 which is engageable with the spring arm 27 adjacent to the fixed contact 26. An adjustment screw 37 threaded into the plate 29 extends through a clearance opening 38 in the spring arm 27 and engages the stop strap 34 adjacent to its free end. With this structure the screw 37 can be threaded in or outto change the position of the end 36 of the stop strap 34 and, in turn, adjust the position of the fixed contact 27 when theswitch is in the position illustrated in FIG. 4. The spring arm 27 is arranged so that it is biased in a direction toward the stop strap and is normally maintained in engagement with the end 36 as illustrated. However, the fixed contact 26 is free to move against the spring bias of the spring arm 27 away from the stop strap when the switch operates.

A double throw mobile contact assembly is provided with opposed contacts 39 and 41, secured to the free end of a mobile contact arm 42. The opposite end of the arm 42 is welded at 43 to a plate member 44. A lateral projection on the plate member 44 serves as the third terminal 46 of the switch. Here again, the terminal 46 is threaded to receive a terminal screw 47. An insulating spacer 48 is positioned between the stop strap 34 and the mobile contact arm 42 to electrically isolate these two members. The structure, however, is arranged so that a single rivet 28 is the only element required to secure the second contact assembly and the mobile contact assembly in the body member 12. The body member 11 is formed with upstanding projections 51 located on opposite sides of each of the terminals 23, 32 and 46 which cooperate with the rivets associated with each of the terminals to insure that the contact assemblies are properly oriented in the body and that the contacts are properly positioned with respect to each other. The tubular extension 52 in the body member 11 extends along the rivet 28 through the mobile contact assembly to insure that the rivet is electrically isolated therefrom.

A rectangular washer 35 formed of fiber or plastic material is loosely located below each terminal in a body recess shaped to prevent washer rotation. Each screw is threaded through the associated washer with a friction fit. These washers function to prevent the screws form shaking out of the device and being lost during shipment of the thermostat. Therefore, the screws 24, 33, and 47 can be assembled during manufacture without being tightened and it is not necessary for the screws to be installed or backed off during wiring of the thermostat to its load.

Mounted in the disc chamber 16 is a smoothly dished bimetallic snap disc 53 and a temperature-adjusting spring assembly 54. Although the illustrated embodiment utilizes a smooth disc, in some instances a disc shaped as disclosed in the pending application of Anton J. Gerich, Ser. No. 859,853, filed Sept. 22, 1969 may be used. The periphery of the disc is seated against a laterally extending annular seat 56 formed in the cover 13. Radial location of the disc is provided by four projecting pins 57. (illustrated in FIG. 3) which extend through mating openings 58 in the cover 13. The seat 56 is provided with a small step 55 to provide a relatively sharp edge for engagement with the disc adjacent its periphery. With this sharp edge structure, the location of the support on the disc is more precisely determined than if a curved seat were employed. The body is provided with a wall 59 proportioned so that it is spaced form the disc to insure that the opposite faces of the disc are not tightly engaged along its periphery. The cover 13 is positioned against an annular end wall 60 which extends around the disc excepting along a radial extension 16a.

The cover 13 is secured to the body by mounting means arranged to insure a full seating contact between the annular step portion 56 and the end wall 60. This mounting structure includes a lateral flange 61 provided with an opening proportioned to receive a tongue or projection 62 on the body and two rivets 63 and 64. The various elements are proportioned so that the cover 13 can be positioned over the tongue 62 and then pivoted inward to its assembled position and riveted in place with rivets 63 and 64. Such pivotal movement causes bending of the flange 61 as it passes over the tongue and provides a spring force urging the adjacent end of the cover 13 into tight engagement with the end face 60. This mounting arrangement provides four forces applied at spaced locations around the periphery of the step 56 urging full seating against the end face 60. The two rivets provide two locations and the other two locations are at the ends of the tongue 62. Because the step 56 is deformed from the surface of the plate, a rigidity is provided which insures full contact between the step portion and the mating surface 60 and exact location of the cover with respect to the body. Since the disc 53 is positioned against the cover it is also located accurately with respect to the body and, in turn. the switch.

A bumper 66 provides an operative connection between the disc 53 and the mobile contact am 42. The bumper engages the disc along its center line 67 at one end and a dimple or projection 68 formed in the arm 42 at its other end. The wall 17 of the body is formed with an opening 69 through which the bumper 66 projects and which guides the bumper for longitudinal movement along the center line 67.

The spring assembly 54 includes an elongated leaf spring 71 provided with a lateral flange 72 at one end. The edge of the flange 72 is located in a V-notch 73 in the body 11 to provide a substantially frictionless pivotal connection for the end of the leaf spring body. Sufficient length is provided at this pivot to insure that the spring is oriented properly toward the disc. The opposite or operative end of the spring is provided with a forked structure having a Vd contact section 74 in engagement with diametrically opposite sides of a flange 75 formed on the bumper. The various elements are proportioned so that the engagement between the Vd ends 74 and the flange 75 causes a force on the bumper in alignment with the center line 67 so eccentric spring forces are not applied to the bumper 66.

An adjustment assembly 76 is mounted in the body and includes a mounting sleeve 77 which is threaded to receive a tubular mounting stem 78. The mounting stem is internally threaded and a calibration screw 79 is threaded therein. The end of the calibration screw 79 engages the spring 71 along the line of action 81. An indicator arm 82 is press fitted onto the stem 78. During the manufacture of the thermostat the stem 78 is positioned at some calibration temperature and the calibration screw 79 is threaded in or out of the stem as required until the operating temperature of the thermostat equals the calibration temperature. After manufacture the calibration screw 79 is normally not again adjusted. A spring clip 83 serves as drag clutch to resist rotation of the stem 78 and to assist in retaining the stem in its adjusted position.

The spring 54 and its associated structure is proportioned so that the movement of the end thereof at the bumper 66 approaches a straight line in alignment with the axis 67. This minimizes any lateral movement which would tend to cause friction when the disc operated and thereby minimizes any tendency for friction induced inaccuracies of operation to occur. Preferably, the pivot point at the V-notch 73 is in substantial alignment with the engaged surface of this flange 75 and the point of engagement with the screw 79 is located, when in the middle of its range of adjustment, so that the portion of the spring adjacent to the operating end at 74 is substantially perpendicular to the center line 67. With the illustrated spring 71 this tends to cause the movement of the contact section to be aligned with the center line 67.

In operation, the bumper 66 is biased toward the disc 53 in all positions of the disc. When the disc is in the position illustrated in FIG. 4, it is curved toward the cover 13 and the mobile contact 39 engages the fixed contact 18. At this time an electrical connection is provided between the terminal 46 and the terminal 23. The various elements are sized so that a spacing exists between the projection 68 and the adjacent end of the bumper at this time.

When a temperature is reached at which the disc starts to snap from the position of FIG. 4 to its opposite position of curvature, the center portion of the disc moves toward the mobile arm and causes the end of the bumper 66 to engage the projection 68 on the mobile arm. This does not occur, however, until the disc is in snap motion. Therefore, full contact pressure is maintained until the instant the contacts open. As the disc continues to snap through to the opposite position of curvature, the mobile contact is moved upward causing the contacts 39 and 18 to separate and causing the contact 41 to engage the fixed contact 26. This occurs before the disc snap movement is completed and when the disc reaches its opposite position of stability, the two contacts 26 and 41 are in engagement and the spring arm 27 is deflected slightly away from the end of the stop strap 34. With this structure there is no positive restraint to limit this travel, therefore the snap range of the disc is a snap range determined by the manufacture of the disc itself. During the movement of the disc from the position of FIG. 4 to its opposite position of stability, the adjusting spring 71 is deflected. However, the absolute spring rate of the adjusting spring is less than the absolute value of the negative spring rate of the disc so snap action is obtained.

When the disc reaches its second operating temperature, it starts to move with snap action toward the position of FIG. 4. During the first portion of this movement, until the arm 27 engages the end of the stop strap 34 at 36, the two contacts move together. Because the supports of the two contacts 26 and 41 tend to .cause them to move along different paths a wiping action occurs as they move together. Further, since the spring arm 27 urges the contact 26 toward the contact 41, contact pressure is maintained until the instant the contacts open. As Soon as the end 36 of the stop strap 34 is engaged, the contact 26 cannot continue to move with the contact 41 and the upper switch opens. Of course, the movement back to the position of FIG. 4causes the contact 39 to engage the fixed contact 18 before the snap movement is completed, so very rapid closing of the two contacts 18 and 39 is achieved.

Preferably, the disc 53 moves to the position of FIG. 4 on decreasing temperature to its lower operating temperature and moves to its opposite position on increasing temperature to its upper operating temperature. The difference between the two temperatures, of course, is the differential temperature of the thennostat. With such an arrangement, it is preferable to connect, the adjacent heater for operation when the disc is in the position of FIG. 4 because the disc is at this time closest to the cover and is therefore most responsive to the'source of heat being sensed. When the upper operating temperature is reached the disc snaps through to its opposite position.

When it is desired to increase the operating temperature of the thermostat the stem 78 is turned in a clockwise direction as viewed in FIG. 1 causing the screw 79 to move down further against the spring. This causes an increase in force to be applied by the spring on the bumper and, in turn, the disc. Such increase in force increases the operating temperatures of the disc but does not materially change the differential temperature of operation. Preferably, the adjustment screw 79 is located so as to engage the spring 71 at a position substantially closer to the picot at 73 than the disc end of the spring. With this structure, the force of the spring on the disc can be changed substantially without excessive movement of the adjusting stem 78 and the spring 77 provides a relatively low spring rate and therefore does not excessively reduce the operating differential of the disc.

It should be understood that if a thermostat requires only a single throw switch the upper fixed contact assembly and the contact 41 may be eliminated. In such a thermostat, however, the functional operation of the thermostat is substantially identical with the thermostat illustrated.

Because the disc is not positively restrained, the location of its snap range is not altered when the operating temperature is changed. Therefore, the operation of the switch is not affected by temperature adjustment. Also, since the line of action 81 of the adjusting screw is perpendicular to a line between the two ends of the leaf spring, adjustment of the leaf spring does not materially alter the spring rate thereof. Consequently, adjustment of the thermostats operating temperature does not alter the differential temperature of the disc.

Because full contact pressure is maintained at all times while the contacts are in engagement and because the contact separation occurs substantially simultaneously with the reduction in contact pressure, the tendency of the contacts to arc either on opening or closing is minimized and the switch can control circuits involving relatively large current without excessive contact deterioration. Further, since the disc and the bumper 66 is in snap motion before the contacts are separated, a substantialmass in motion is available to break any welds between the contacts which may occur. Also, the complete separation of the switch chamber from the disc chamber isolates the disc and the adjusting spring form contamination caused by are splatter.

Although a preferred embodiment of this invention is illustrated, it is to be understood that various modifications and rearrangements of parts may be resorted to without departing from the scope of the invention claimed herein.

1. An adjustable thermostat comprising a body assembly, switch means mounted on said body assembly including a first fixed contact and a mobile contact movable toward and away from said fixed contact, a bimetallic snap disc on said body assembly operable to snap along a line of action in response to temperature changes, operating means connecting said disc and mobile contact for opening and closing said switch means only after said disc is in snap motion, said thermostat being substantially free of positive limits restricting the movement of said disc, an adjustable spring means operable to apply an adjustable biasing force to said disc and adjustably determine the operating temperature of said disc, said adjustable spring means including a leaf spring element supported on said body assembly at a first location spaced from said disc, said spring element providing an operating portion at a second location spaced from said first location applying a force to said disc along said line of action thereof in all positions of said disc, and adjusting means spaced from both said first and second 10- cations operating to adjust the force of said spring applied to said disc, said adjusting means being located closer to said first location than to said second location.

2. An adjustable thermostat as set forth in claim 1 wherein said operating means is an elongated bumper guided in said body assembly forlongitudinal movement along said line of action, said bumper being provided with a radial surface spaced from said disc and substantially symmetrical with respect to said line of action, said spring element engaging said surface to apply said force to said disc.

3. A thermostat as set forth in claim 2 wherein said body assembly includes a wall separating said switch means from said disc and said adjustable spring means, said wall being provided with an opening therethrough, said bumper extending through and being guided by said opening, said bumper and wall cooperating to isolate said switch means from said disc and said adjustable spring means.

4. An adjustable thermostat as set forth in claim 2 wherein said spring element provides a free end engaging said surface.

5. An adjustable thermostat as set forth in claim 4 wherein pivot means connects said spring element to said body assembly at said first location.

6. An adjustable thermostat as set forth in claim SWherein said pivot means includes a lateral projection on said spring element having an edge supported in a notch in said body assembly.

7. An adjustable thermostat as set forth in claim 6 wherein adjusting means provides an abutting surface engaging said spring element intermediate said first and second locations.

8. An adjustable thermostat as set forth in claim 7 wherein said abutting surface is adjustable relative to said body assembly in a direction substantially parallel to said line of action.

9. An adjustable thermostat as set forth in claim 8 wherein the section of said spring adjacent to said second location extends substantially perpendicular to said line of action and moves in response to movement of said disc in a direction substantially parallel to said line of action.

10. An adjustable thermostat as set forth in claim 9 wherein said body assembly includes a metal cover formed with a circular edge, and said disc is supported adjacent to its periphery by said circular edge when said disc is in at least one position of stability.

11. An adjustable thermostat as set forth in claim 10 wherein said cover is provided with an offset portion radially inward from said edge which provides clearance with respect to said disc.

12. An adjustable thermostat as set forth in claim 11 wherein said disc moves to a position in which it is curved toward said offset portion and is closely adjacent thereto when the temperature of said disc drops to its lower operating temperature. I

13. An adjustable thermostat as set forth in claim 12 wherein said switch means includes a second fixed contact on the side of said mobile contact opposite said first fixed contact, and said disc maintains said mobile contact in engagement with one of said fixed contacts when said disc is in one position of stability and maintains said mobile contact in engagement with the other of said fixed contacts when said disc is in the other position of stability.

14. An adjustable thermostat as set forth in claim 13 wherein said switch means includes terminal means provided with terminal screws, and friction means engage said terminal screws resisting rotation thereof.

15. An adjustable thermostat as set forth in claim 1 wherein said body assembly includes a metal cover formed with a circular edge, and said disc is supported adjacent to its periphery by said circular edge when said disc is in at least one position of stability.

16. An adjustable snap-acting thermostat comprising a housing assembly, switch means in said housing assembly including a fixed contact and a mobile contact movable into and out of engagement with said fixed contact when said switch means is operated, a bimetallic snap element in said body movable with snap movement along a line of action between first and second positions of stability in response to temperature changes, operator means connected between said snap element and mobile contact operating said switch means in response to movement of said snap element only after said snap element is in snap movement, adjustable spring means operable to change the operating temperature of said snap element without positively restricting the movement thereof. said spring means including a leaf spring supported at one end on said housing assembly and providing at its other end an operating portion movable substantially parallel to said line of action and applying an adjustable spring force to said snap element substantially along said line of action urging said snap element toward one of said positions of stability, and adjusting means intermediate the ends of said spring operable to change the value of said spring force and the operating temperature of said snap element.

17. An adjustable thermostat as set forth in claim 16 wherein said adjustment means engages said spring substantially closer to said one end than said other end.

18. An adjustable thermostat as set forth in claim 16 wherein said body assembly includes a body member and a metallic cover, said disc being seated at its periphery against said cover, said body member being provided with an annular locating surface engaging and positioning said cover, said body member providing a tongue, said cover providing a strap section extending over said tongue and being deflected thereby to cause a force at two locations urging said cover against said surface, and fastener means cooperating with said tongue to secure said cover on said body. 

1. An adjustable thermostat comprising a body assembly, switch means mounted on said body assembly including a first fixed contact and a mobile contact movable toward and away from said fixed contact, a bimetallic snap disc on said body assembly operable to snap along a line of action in response to temperature changes, operating means connecting said disc and mobile contact for opening and closing said switch means only after said disc is in snap motion, said thermostat being substantially free of positive limits restricting the movement of said disc, an adjustable spring means operable to apply an adjustable biasing force to said disc and adjustably determine the operating temperature of said disc, said adjustable spring means including a leaf spring element supported on said body assembly at a first location spaced from said disc, said spring element providing an operating portion at a second location spaced from said first location applying a force to said disc along said line of action thereof in all positions of said disc, and adjusting means spaced from both said first and second locations operating to adjust the force of said spring applied to said disc, said adjusting means being located closer to said first location than to said second location.
 2. An adjustable thermostat as set forth in claim 1 wherein said operating means is an elongated bumper guided in said body assembly for longitudinal movement along said line of action, said bumper being provided with a radial surface spaced from said disc and substantially symmetrical with respect to said line of action, said spring element engaging said surface to apply said force to said disc.
 3. A thermostat as set forth in claim 2 wherein said body assembly includes a wall separating said switch means from said disc and said adjustable spring means, said wall being provided with an opening therethrough, said bumper extending through and being guided by said opening, said bumper and wall cooperating to isolate said switch means from said disc and said adjustable spring means.
 4. An adjustable thermostat as set forth in claim 2 wherein said spring element provides a free end engaging said surface.
 5. An adjustable thermostat as set forth in claim 4 wherein pivot means connects said spring element to said body assembly at said first location.
 6. An adjustable thermostat as set forth in claim 5 wherein said pivot means includes a lateral projection on said spring element having an edge supported in a notch in said body assembly.
 7. An adjustable thermostat as set forth in claim 6 wherein adjusting means provides an abutting surface engaging said spring element intermediate said first and second locations.
 8. An adjustable thermostat as set forth in claim 7 wherein said abutting surface is adjustable relative to said body assembly in a direction substantially parallel to said line of action.
 9. An adjustable thermostat as set forth in claim 8 wherein the section of said spring adjacent to said second location extends substantially perpendicular to said line of action and moves in response to movement of said disc in a direction substantially parallel to said line of action.
 10. An adjustable thermostat as set forth in claim 9 wherein said body assembly includes a metal cover formed with a circular edge, and said disc is supported adjacent to its periphery by said circular edge when said disc is in at least one position of stability.
 11. An adjustable thermostat as set forth in claim 10 wherein said cover is provided with an offset portion radially inward from said edge which provides clearance with respect to said disc.
 12. An adjustable thermostat as set forth in claim 11 wherein said disc moves to a position in which it is curved toward said offset portion and is closely adjacent thereto when the temperature of said disc drops to its lower operating temperature.
 13. An adjustable thermostat as set forth in claim 12 wherein said switch means includes a second fixed contact on the side of said mobile contact opposite said first fixed contact, and said disc maintains said mobile contact in engagement with one of said fixed contacts when said disc is in one position of stability and maintains said mobile contact in engagement with the other of said fixed contacts when said disc is in the other position of stability.
 14. An adjustable thermostat as set forth in claim 13 wherein said switch means includes terminal means provided with terminal screws, and friction means engage said terminal screws resisting rotation thereof.
 15. An adjustable thermostat as set forth in claim 1 wherein said body assembly includes a metal cover formed with a circular edge, and said disc is supported adjacent to its periphery by said circular edge when said disc is in at least one position of stability.
 16. An adjustable snap-acting thermostat comprising a housing assembly, switch means in said housing assembly including a fixed contact and a mobile contact movable into and out of engagement with said fixed contact when said switch means is operated, a bimetallic snap element in said body movable with snap movement along a line of action between first And second positions of stability in response to temperature changes, operator means connected between said snap element and mobile contact operating said switch means in response to movement of said snap element only after said snap element is in snap movement, adjustable spring means operable to change the operating temperature of said snap element without positively restricting the movement thereof, said spring means including a leaf spring supported at one end on said housing assembly and providing at its other end an operating portion movable substantially parallel to said line of action and applying an adjustable spring force to said snap element substantially along said line of action urging said snap element toward one of said positions of stability, and adjusting means intermediate the ends of said spring operable to change the value of said spring force and the operating temperature of said snap element.
 17. An adjustable thermostat as set forth in claim 16 wherein said adjustment means engages said spring substantially closer to said one end than said other end.
 18. An adjustable thermostat as set forth in claim 16 wherein said body assembly includes a body member and a metallic cover, said disc being seated at its periphery against said cover, said body member being provided with an annular locating surface engaging and positioning said cover, said body member providing a tongue, said cover providing a strap section extending over said tongue and being deflected thereby to cause a force at two locations urging said cover against said surface, and fastener means cooperating with said tongue to secure said cover on said body. 